1. Field of the Invention
This invention relates to highly oriented, partially crystalline films of oxides of molybdenum, tungsten and titanium and to sensors for reducing gases such as hydrogen made thereof.
2. Description of Related Art
The production of single molecular thickness layers of MoS.sub.2 and other transition metal dichalcogenides is disclosed in U.S. Pat. No. 4,822,590 to S. Roy Morrison et al. The MoS.sub.2 is exfoliated into monolayers by intercalation with lithium followed by reaction with water. The reaction between the water and the lithium forces the layers of MoS.sub.2 apart into one-molecule-thick platelets. Flocculation occurred rapidly when the pH was reduced to a value of 3 or less.
U.S. Pat. No. 4,996,108 to Divigalpitya et al carries this process further by forming a suspension of a transition metal dichalcogenide in water and adding to the suspension a liquid which is immiscible with water. The mixture is agitated, then allowed to rest. A sheet-like composition of about 500 .ANG. to 750 .ANG. forms at the interface of the water and the immiscible liquid.
In U.S. Pat. No. 4,853,359 to Morrison et al., a novel flocculated, supported single-layered transition metal dichalcogenide catalyst is disclosed. The patent discloses that if the pH is maintained at a value above about 3, the suspension of MoS.sub.2 will not flocculate, but will stay for days or more suspended in water. The next step in producing the catalyst according to the patent is to add a catalytic promoter to the suspension. A solid is formed by adding this promoter or support substance to the water. The flocculated solid is then separated from the liquid.
In an Article, J. Appl. Phys. 69(9), 6373 (1991) by Miremadi et al., the deposition of thick oriented films of a transition metal dichalcogenide, with a thickness in the order of 0.1 to 10 microns is disclosed.
In pending U.S. patent application Ser. No. 07/704,432, a novel transition metal dichalcogenide with a house-of-cards (HOC) structure is disclosed and a method for production thereof. The method involves reducing the pH of a suspension of single-molecular thickness platelets of MoS.sub.2, or other transition metal dichalcogenides, until they flocculate, and the pH is within a range between a first pH where basal planes of the platelets have a zero charge and a second pH where edges of the platelets have a zero charge.
The use of the semiconductor metal oxides MoO.sub.3 or WO.sub.3 in gas sensors using thermal evaporation, sputtering and some impregnation techniques is described in the prior art. For example, in Japanese Patent No. 61-118651 to Ayusawa, an evaporated film of WO.sub.3 is used as a hydrogen gas sensor. The same patent describes a rather lengthy chemical impregnation process for deposition of polycrystalline MoO.sub.3 powder, using binders and adhesives, although the MoO.sub.3 data is not presented.
Films made of oxides of molybdenum, tungsten and titanium have been produced on a substrate by sintering highly oriented sulfide films of the same metals. However, this is done at temperatures above 350.degree. C., resulting in highly crystallized films where the particles tend to migrate to form much larger particles. The oxide particles lose the previous high orientation of the sulfide film and sensors prepared therefrom have little or no sensitivity to hydrogen.
Sensors have also been prepared from commercial oxide powders of the same metals or from the sulfide powders of the same metals which have been impregnated with platinum and sintered above 350.degree. C. These sensors show relatively poor sensitivity to hydrogen.